.TH IPERF3 1 "November 2025" ESnet "User Manuals"
.SH NAME
iperf3 \- perform network throughput tests
.SH SYNOPSIS
.B iperf3 -s [
.I options
.B ]
.br
.B iperf3 -c
.I server
.B [
.I options
.B ]

.SH DESCRIPTION
iperf3 is a tool for performing network throughput measurements.
It can test TCP, UDP, or SCTP throughput.
To perform an iperf3 test the user must establish both a server and a
client.
.PP
The iperf3 executable contains both client and server functionality.
An iperf3 server can be started using either of the -s or
--server command-line parameters, for example:
.IP
\fCiperf3 -s\fR
.IP
\fCiperf3 --server \fR
.PP
Note that many iperf3 parameters have both short (-s) and long
(--server) forms.
In this section we will generally use the short form of command-line
flags, unless only the long form of a flag is available.
.PP
By default, the iperf3 server listens on TCP port 5201 for connections
from an iperf3 client.
A custom port can be specified by using the -p flag, for
example:
.IP
\fCiperf3 -s -p 5002\fR
.PP
After the server is started, it will listen for connections from
iperf3 clients (in other words, the iperf3 program run in client
mode).
The client mode can be started using the -c command-line option,
which also requires a host to which iperf3 should connect.
The host can be specified by hostname, IPv4 literal, or IPv6 literal:
.IP
\fCiperf3 -c iperf3.example.com\fR
.IP
\fCiperf3 -c 192.0.2.1\fR
.IP
\fCiperf3 -c 2001:db8::1\fR
.PP
If the iperf3 server is running on a non-default TCP port, that port
number needs to be specified on the client as well:
.IP
\fCiperf3 -c iperf3.example.com -p 5002\fR
.PP
The initial TCP connection is used to exchange test parameters,
control the start and end of the test, and to exchange test results.
This is sometimes referred to as the "control connection".
The actual test data is sent over a separate TCP connection, as a
separate flow of UDP packets, or as an independent SCTP connection,
depending on what protocol was specified by the client.
.PP
Normally, the test data is sent from the client to the server, and
measures the upload speed of the client.
Measuring the download speed from the server can be done by specifying
the -R flag on the client.
This causes data to be sent from the server to the client.
.IP
\fCiperf3 -c iperf3.example.com -p 5202 -R
.PP
Results are displayed on both the client and server.
There will be at least one line of output per measurement interval (by
default a measurement interval lasts for one second, but this can be
changed by the -i option).
Each line of output includes (at least) the time since the start of
the test, amount of data transferred during the interval, and the
average bitrate over that interval.
Note that the values for each measurement interval are taken from the
point of view of the endpoint process emitting that output (in other
words, the output on the client shows the measurement interval data for
the client.
.PP
At the end of the test is a set of statistics that shows (at
least as much as possible) a summary of the test as seen by both the
sender and the receiver, with lines tagged accordingly.
Recall that by default the client is the sender and the server is the
receiver, although as indicated above, use of the \fC-R\fR flag will
reverse these roles.
.PP
The client can be made to retrieve the server-side output for a given
test by specifying the --get-server-output flag.
.PP
Either the client or the server can produce its output in a JSON
structure, useful for integration with other programs, by passing it
the -J flag.
Normally the contents of the JSON structure are only completely known
after the test has finished, no JSON output will be emitted until the
end of the test.
By enabling line-delimited JSON multiple objects will be emitted to
provide a real-time parsable JSON output.
.PP
iperf3 has a (overly) large set of command-line options that can be
used to set the parameters of a test.
They are given in the "GENERAL OPTIONS" section of the manual page
below, as well as summarized in iperf3's help output, which can be
viewed by running iperf3 with the -h flag.
.SH "GENERAL OPTIONS"
.TP
.BR -p ", " --port " \fIn\fR"
Set server port to listen on/connect to to \fIn\fR (default 5201)
.TP
.BR -f ", " --format " [kmgtKMGT]"
Set format to report: Kbits/Mbits/Gbits/Tbits
.TP
.BR -i ", " --interval " \fIn\fR"
Pause \fIn\fR seconds between periodic throughput reports;
default is 1, use 0 to disable.
.TP
.BR -I ", " --pidfile " \fIfile\fR"
Write a file with the process ID.
This option is most useful when running as a daemon.
.TP
.BR -F ", " --file " \fIname\fR"
Use a file as the source (on the sender) or sink (on the receiver) of
data, rather than just generating random data or throwing it away.
This feature is used for finding whether or not the storage subsystem
is the bottleneck for file transfers.
It does not turn iperf3 into a file transfer tool.
The length, attributes, and in some cases contents of the received
file may not match those of the original file. This option is
unavailable when doing
.B --udp
tests.
.TP
.BR -A ", " --affinity " \fIn/n,m\fR"
Set the CPU affinity, if possible (Linux, FreeBSD, and Windows only).
On both the client and server you can set the local affinity by using
the \fIn\fR form of this argument (where \fIn\fR is a CPU number).
In addition, on the client side you can override the server's
affinity for just that one test, using the \fIn,m\fR form of
argument.
Note that when using this feature, a process will only be bound
to a single CPU (as opposed to a set containing potentially multiple
CPUs).
.TP
.BR -B ", " --bind " \fIhost\fR[\fB%\fIdev\fR]"
Bind to the specific interface associated with address \fIhost\fR.
If an optional interface is specified, it is treated as a shortcut
for \fB--bind-dev \fIdev\fR.
Note that a percent sign and interface device name are required for
IPv6 link-local address literals, in order to set the link-local
scope.
.TP
.BR --bind-dev " \fIdev\fR"
Bind to the specified network interface.
This option uses SO_BINDTODEVICE, and may require root permissions.
(Available on Linux and possibly other systems.)
.TP
.BR -V ", " --verbose " "
Produce more detailed output.
.TP
.BR -J ", " --json " "
Output in JSON format instead of the default human-readable
output.
.TP
.BR --json-stream " "
Output in line-delimited JSON format instead of the default
human-readable output. This option overrides the
.B --json
option, if that option was also specified.
.TP
.BR --json-stream-full-output " "
Output in JSON format with JSON streams enabled. This flag only takes
effect if the
.B --json-stream
option was also specified.
.TP
.BR --logfile " \fIfile\fR"
Send output to a log file.
.TP
.BR --forceflush " "
Force flushing output at every interval.
Used to avoid buffering when sending output to pipe.
.TP
.BR --timestamps "[\fB=\fIformat\fR]"
Prepend a timestamp at the start of each output line.
By default, timestamps have the format emitted by
.BR ctime ( 1 ).
Optionally, \fC=\fR followed by
a format specification can be passed to customize the
timestamps, see
.BR strftime ( 3 ).
If this optional format is given, the \fC=\fR must immediately
follow the \fB--timestamps\fR option with no whitespace intervening.
.TP
.BR --rcv-timeout " \fI#\fR"
Set idle timeout for receiving data during active tests. The receiver
will halt a test if no data is received from the sender for this
number of ms (default to 120000 ms, or 2 minutes).
.TP
.BR --snd-timeout " \fI#\fR"
Set timeout for unacknowledged TCP data (on both test and control
connections) This option can be used to force a faster test timeout
in case of a network partition during a test. The required
parameter is specified in ms, and defaults to the system settings.
This functionality depends on the TCP_USER_TIMEOUT socket option, and
will not work on systems that do not support it.
.TP
.BR --use-pkcs1-padding
This option is only meaningful when using iperf3's authentication
features. Versions of iperf3 prior to 3.17 used PCKS1 padding in the
RSA-encrypted credentials, which was vulnerable to a side-channel
attack that could reveal a server's private key.  Beginning with
iperf-3.17, OAEP padding is used, however this is a breaking change
that is not compatible with older iperf3 versions.  Use this option to
preserve the less secure, but more compatible, behavior.
.TP
.BR -m ", " --mptcp " "
Use the MPTCP variant for the current protocol. This only applies to
TCP and enables MPTCP usage.
.TP
.BR -d ", " --debug " "
Emit debugging output.
Primarily (perhaps exclusively) of use to developers.
.TP
.BR -v ", " --version " "
Show version information and quit.
.TP
.BR -h ", " --help " "
Show a help synopsis.

.SH "SERVER SPECIFIC OPTIONS"
.TP
.BR -s ", " --server " "
Run in server mode.
.TP
.BR -D ", " --daemon " "
Run the server in background as a daemon.
.TP
.BR -1 ", " --one-off
Handle (at most) one client connection, then exit.
If an idle time is set, the server will exit after that amount of time
with no connection.
.TP
.BR --idle-timeout " \fIn\fR"
Restart the server after \fIn\fR seconds in case it gets stuck.
In one-off mode, this is the number of seconds the server will wait
before exiting.
.TP
.BR --server-max-duration " \fIn\fR"
The maximum time, in seconds, that an iperf client can run against the server.
When the sum of the client's time and omit values exceeds the max duration set by the server
or the client's time value is 0, the measurement is rejected.
.TP
.BR --server-bitrate-limit " \fIn\fR[KMGT][/\fCn\fR]"
Set a limit on the server side, which will cause a test to abort if
the client specifies a test of more than \fIn\fR bits per second, or
if the average data sent or received by the client (including all data
streams) is greater than \fIn\fR bits per second.  The default limit
is 0, which implies no limit.  The interval over which to average the
data rate is 5 seconds by default, but can be specified by adding a
.B
/
character and a number to the bitrate specifier.
.TP
.BR --rsa-private-key-path " \fIfile\fR"
Path to the RSA private key (not password-protected) used to decrypt
authentication credentials from the client (if built with OpenSSL
support).
.TP
.BR --authorized-users-path " \fIfile\fR"
Path to the configuration file containing authorized users credentials
to run iperf tests (if built with OpenSSL support).  The file is a
comma separated list of usernames and password hashes; more
information on the structure of the file can be found in the EXAMPLES
section.
.TP
.BR --time-skew-threshold " \fIseconds\fR"
Specify the allowable time skew threshold (in seconds) between the
server and client during the authentication process.
.SH "CLIENT SPECIFIC OPTIONS"
.TP
.BR -c ", " --client " \fIhost\fR[\fB%\fIdev\fR]"
Run in client mode, connecting to the specified server.
By default, a test consists of sending data from the client to the
server, unless the \-R flag is specified.
If an optional interface is specified, it is treated as a shortcut
for \fB--bind-dev \fIdev\fR.
Note that a percent sign and interface device name are required for
IPv6 link-local address literals.
.TP
.BR --sctp
Use SCTP for tests rather than TCP (FreeBSD and Linux).
Note that TCP communication is still used for the control connection
between client and server.
.TP
.BR -u ", " --udp
Use UDP for tests rather than TCP.
Note that TCP communication is still used for the control connection
between client and server.
.TP
.BR --connect-timeout " \fIn\fR"
Set timeout for establishing the initial control connection to the
server, in milliseconds.
The default behavior is the operating system's timeout for TCP
connection establishment.
Providing a shorter value may speed up detection of a down iperf3
server.
.TP
.BR -b ", " --bitrate " \fIn\fR[KMGT]"
Set target bitrate to \fIn\fR bits/sec (default 1 Mbit/sec for UDP,
unlimited for TCP/SCTP).
If there are multiple streams (\-P flag), the throughput limit is applied
separately to each stream.
You can also add a '/' and a number to the bitrate specifier.
This is called "burst mode".
It will perform the given number of sends without pausing,
even if that
temporarily exceeds the specified throughput limit.
Setting the target bitrate to 0 will disable bitrate limits
(particularly useful for UDP tests).
This throughput limit is implemented internally inside iperf3, and is
available on all platforms.
Compare with the
.B --fq-rate
flag.
This option replaces the
.B --bandwidth
flag, which is now deprecated
but (at least for now) still accepted.
.TP
.BR --pacing-timer " \fIn\fR[KMGT]"
Set pacing timer interval in microseconds (default 1000 microseconds,
or 1 ms).
This controls iperf3's internal pacing timer for the \-b/\--bitrate
option.
The timer fires at the interval set by this parameter.
Smaller values of the pacing timer parameter smooth out the traffic
emitted by iperf3, but potentially at the cost of performance due to
more frequent timer processing.
.TP
.BR --fq-rate " \fIn\fR[KMGT]"
Set a rate to be used with fair-queueing based socket-level pacing,
in bits per second.
This pacing (if specified) will be in addition to any pacing due to
iperf3's internal throughput pacing (\-b/\--bitrate flag), and both can be
specified for the same test.
Only available on platforms supporting the
\fCSO_MAX_PACING_RATE\fR socket option (currently only Linux).
The default is no fair-queueing based pacing.
.TP
.BR --no-fq-socket-pacing
This option is deprecated and will be removed.
It is equivalent to specifying
.BR
--fq-rate=0 .
.TP
.BR -t ", " --time " \fIn\fR"
Set the test duration in seconds (default 10 secs).
The
.BR
-t , -n ", and" -k
options are mutually exclusive.
.TP
.BR -n ", " --bytes " \fIn\fR[KMGT]"
Set the number of bytes to transmit.
The
.BR
-t , -n ", and" -k
options are mutually exclusive.
.TP
.BR -k ", " --blockcount " \fIn\fR[KMGT]"
Set the number of blocks (packets) to transmit.
The
.BR
-t , -n ", and" -k
options are mutually exclusive.
.TP
.BR -l ", " --length " \fIn\fR[KMGT]"
Set the length of the buffer to read or write.  For TCP tests, the
default value is 128KB.
In the case of UDP, iperf3 tries to dynamically determine a reasonable
sending size based on the path MTU; if that cannot be determined it
uses 1460 bytes as a sending size.
For SCTP tests, the default size is 64KB.
.TP
.BR --cport " \fIport\fR"
Bind data streams to a specific TCP or UDP client port (for TCP
and UDP only, default is to use an ephemeral port).
.TP
.BR -P ", " --parallel " \fIn\fR"
Set the number of parallel client streams to run. Beginning with
iperf-3.16, iperf3 will spawn off a separate thread for each test
stream.
Using multiple streams may result in higher throughput than a
single stream, in cases where network throughput is CPU-limited.
.TP
.BR -R ", " --reverse
Reverse the direction of a test, so that the server sends data to the
client.
.TP
.BR --bidir
Test in both directions (normal and reverse), with both the client and
server sending and receiving data simultaneously
.TP
.BR -w ", " --window " \fIn\fR[KMGT]"
Set t he socket buffer size / window size.
This value gets sent to the server and used on that side too;
on both sides this option sets both the sending and receiving
socket buffer sizes.
This option can be used to set (indirectly) the maximum TCP window
size.
Note that on Linux systems, the effective maximum window size is
approximately
double what is specified by this option.
This behavior is not a bug in iperf3 but a feature of the
Linux kernel, as documented by
.BR tcp ( 7 )
and
.BR socket ( 7 )).
.TP
.BR -M ", " --set-mss " \fIn\fR"
Set the TCP/SCTP maximum segment size (MTU - 40 bytes).
.TP
.BR -N ", " --no-delay " "
Set the TCP/SCTP no delay option, disabling Nagle's Algorithm.
.TP
.BR -4 ", " --version4 " "
Force the use of IPv4.
.TP
.BR -6 ", " --version6 " "
Force the use of IPv6.
.TP
.BR -S ", " --tos " \fIn\fR"
Set the IP type of service bits.
The usual prefixes for octal and hex can be used,
i.e. 52, 064 and 0x34 all specify the same value.
.TP
.BR "--dscp " \fIdscp\fR
Set the IP DSCP bits.  Both numeric and symbolic values are accepted. Numeric
values can be specified in decimal, octal and hex (see
.B --tos
above).
.TP
.BR -L ", " --flowlabel " \fIn\fR"
Set the IPv6 flow label (currently only supported on Linux).
.TP
.BR -X ", " --xbind " \fIname\fR"
Bind SCTP associations to a specific subset of links using sctp_bindx(3).
The \fB--B\fR flag will be ignored if this flag is specified.
Normally SCTP will include the protocol addresses of all active links
on the local host when setting up an association. Specifying at least
one \fB--X\fR name will disable this behaviour.
This flag must be specified for each link to be included in the
association, and is supported for both iperf servers and clients
(the latter are supported by passing the first \fB--X\fR argument to bind(2)).
Hostnames are accepted as arguments and are resolved using
getaddrinfo(3).
If the
.B --4
or
.B --6
flags are also specified, names
which do not resolve to addresses within the
specified protocol family will be ignored.
.TP
.BR --nstreams " \fIn\fR"
Set number of SCTP streams.
.TP
.BR -Z ", " --zerocopy " "
Use a "zero copy" method of sending data, such as sendfile(2),
instead of the usual write(2).
.TP
.BR --skip-rx-copy
Ignored received packet data, using the MSG_TRUNC flag to the
recv(2) system call.
.TP
.BR -O ", " --omit " \fIn\fR"
Perform pre-test for
.I n
seconds and omit the pre-test statistics, to skip past the TCP slow-start
period.
.TP
.BR -T ", " --title " \fIstr\fR"
Prefix every output line with the string
.IR str .
.TP
.BR --extra-data " \fIstr\fR"
Specify an extra data string field to be included in JSON output.
.TP
.BR -C ", " --congestion " \fIalgo\fR"
Set the congestion control algorithm (Linux and FreeBSD only).  An
older
.B --linux-congestion
synonym for this flag is accepted but is deprecated.
.TP
.BR --get-server-output
Get the output from the server.
The output format is determined by the server (in particular, if the
server was invoked with the \fB--json\fR flag, the output will be in
JSON format, otherwise it will be in human-readable format).
If the client is run with \fB--json\fR, the server output is included
in a JSON object; otherwise it is appended at the bottom of the
human-readable output. Note that the server output is available only
if the test completes, not if it is interrupted.
.TP
.BR --udp-counters-64bit
Use 64-bit counters in UDP test packets.
The use of this option can help prevent counter overflows during long
or high-bitrate UDP tests.  Both client and server need to be running
at least version 3.1 for this option to work.  It may become the
default behavior at some point in the future.
.TP
.BR --repeating-payload
Use repeating pattern in payload, instead of random bytes.
The same payload is used in iperf2 (ASCII '0..9' repeating).
It might help to test and reveal problems in networking gear with hardware
compression (including some WiFi access points), where iperf2 and iperf3
perform differently, just based on payload entropy.
.TP
.BR --dont-fragment
Set the IPv4 Don't Fragment (DF) bit on outgoing packets.
Only applicable to tests doing UDP over IPv4.
.TP
.BR --username " \fIusername\fR"
Specify username to use for authentication to the iperf server
(if built with OpenSSL support).
The password will be prompted for interactively when the test is run.
Note the password can also be specified via the IPERF3_PASSWORD
environment variable. If this variable is present, the password
prompt will be skipped.
.TP
.BR --rsa-public-key-path " \fIfile\fR"
Set path to the RSA public key used to encrypt authentication
credentials
(if built with OpenSSL support).

.SH EXAMPLES
.SS "Authentication - RSA Keypair"
The authentication feature of iperf3 requires an RSA public keypair.
The public key is used to encrypt the authentication token containing the
user credentials, while the private key is used to decrypt the authentication token.
The private key must be in PEM format and additionally must not have a
password set.
The public key must be in PEM format and use SubjectPrefixKeyInfo encoding.
An example of a set of UNIX/Linux commands using OpenSSL
to generate a correctly-formed keypair follows:
.IP
\fC> openssl genrsa -des3 -out private.pem 2048\fR
.sp 0
\fC> openssl rsa -in private.pem -outform PEM -pubout -out public.pem\fR
.sp 0
\fC> openssl rsa -in private.pem -out private_not_protected.pem \\ \fR
.sp 0
\fC  -outform PEM\fR
.PP
After these commands, the public key will be contained in the file
public.pem and the private key will be contained in the file
private_not_protected.pem.
.SS "Authentication - Authorized users configuration file"
A simple plaintext file must be provided to the iperf3 server in order to specify
the authorized user credentials.
The file is a simple list of comma-separated pairs of a username and a
corresponding password hash.
The password hash is a SHA256 hash of the string "{$user}$password".
The file can also contain commented lines (starting with the \fC#\fR
character).
An example of commands to generate the password hash on a UNIX/Linux system
is given below:
.IP
\fC> S_USER=mario S_PASSWD=rossi\fR
.sp 0
\fC> echo -n "{$S_USER}$S_PASSWD" | sha256sum | awk '{ print $1 }'\fR
.PP
An example of a password file (with an entry corresponding to the
above username and password) is given below:
.IP
\fC> cat credentials.csv\fR
.in -.5i
.sp 0
\fC# file format: username,sha256\fR
.sp 0
\fCmario,bf7a49a846d44b454a5d11e7acfaf13d138bbe0b7483aa3e050879700572709b\fR
.in +.5i
.SH AUTHORS
A list of the contributors to iperf3 can be found within the
documentation located at
\fChttps://software.es.net/iperf/dev.html#authors\fR.

.SH "SEE ALSO"
libiperf(3),
https://software.es.net/iperf
